There are several types of Automatic Protection Switching (APS) defined, in particular 1+1. In 1+1 APS, there are two sets of ports, the transmit primary and receive primary ports, and the transmit secondary and receive secondary ports, with each set of ports connected by a separate physical link. Note that since transmission is bi-directional, each port is simultaneously both a transmit and a receive port. For clarity, however, only one direction will be described.
Data is transmitted by both the primary and secondary transmit ports over two physical links concurrently, but is only received by receive primary port.
There are a variety of soft and hard errors associated with the SONET physical level which will cause APS to trigger a switch from primary to secondary. Soft errors include counts of corrupted data received while hard errors include loss of carrier. When such an error occurs, the receive secondary port is converted to the receive primary port and the receive primary port is converted to the receive secondary port.
In 1+1 unidirectional APS, this is the extent of the protocol, and the transmitting ports are not aware that anything has happened. In 1+1 bidirectional APS, when the receive side switches from the primary to the secondary port, it uses the SONET K1 and K2 bytes in its transmit direction to inform the other side of its decision. The other side is then free to switch its receive port from primary to secondary. This is useful, for example, in the case where the primary physical link has been cut and one side detects the error before the other.
In order to provide 1+1 port redundancy for data connections, it is necessary as part on the establishment of a connection to establish redundant paths through a switch for both directions of the connection. In particular, a connection will have primary and secondary ports for the incoming link as well as primary and secondary ports for the outgoing link, and these need to be cross connected. The terms “incoming” and “outgoing” are used to indicate the link that a connection setup message is received on and the link that the connection setup message is transmitted on, respectively. I.e., there needs to be a path from the incoming primary port to the outgoing primary and secondary ports and a path from the incoming secondary port to the outgoing primary and second ports, as well as a path from the outgoing primary port to the incoming primary and secondary ports and a path from the outgoing secondary port to the incoming primary and secondary ports.
Typically, this would be done by establishing a mesh of point-to-point paths through the switch and replicating data traffic at the point of ingress. For example, data received on the incoming primary port would have to be replicated and copies sent to the outgoing primary and secondary ports.
The present invention replaces the mesh of point-to-point paths with a set of point-to-multipoint paths through the switch, taking advantage of the multicast capabilities of a switch. The present invention improves on typical implementations of 1+1 port redundancy in three ways:
It provides for faster connection setup because four pt-mpt paths are established through the switch rather than eight pt-pt paths.
A single copy of received data transits the switch rather than two copies. This allows unused switch bandwidth to be used for additional non-redundant ports.
It provides for faster switchover because the switchover does not need to be coordinated between the receive and transmit ports within a switch.